According to the Federal Communications Commission (FCC) cellular radiotelephone calls must be geographically locatable by the year 2001. This capability is desirable for emergency systems such as E911. The FCC requires stringent accuracy and availability performance objectives and demands that cellular radiotelephones be locatable within 125 meters 67% of the time. This threshold has been difficult to achieve using traditional TOA/TDOA (Time Of Arrival/Time Difference Of Arrival) infrastructure technology.
In order to include GPS in wireless portable devices such as cellular radiotelephones, performance needs to be improved in several areas including weak signal detection, acquisition time and energy use for operating power. Regarding weak signal detection, users of cellular radiotelephones have become accustomed to making calls indoors, and traditional processing of GPS signals will not accommodate the attenuation caused by most buildings. Since these GPS receivers capture signals from satellites at quite an extraordinary distance, any objects in the direct line of sight between the GPS receiver and the satellites often cause malfunction because the signal transmitted by the satellites is attenuated by the interfering object making it difficult for the GPS receiver to receive them. Trees, buildings, and other high-profile objects can cause line of sight interference resulting in the problem of weak, or low signal detection.
Regarding accuracy, differential GPS approaches may work but are complex and costly. Moreover they don't fix the weak signal problem.
A major problem with traditional GPS signal processing techniques has to do with bandwidth and signal power. The GPS satellites transmit a very weak signal, guaranteed signal levels are only -130 dBm on the surface of the earth. Actual signals as measured on the earth's surface show signal levels of about -125 dBm. The acquisition threshold of current automotive and consumer grade handheld GPS receivers is on the order of -137 dBm, thus the link margin for signal acquisition is only about 7 to 12 dB.
The sequential detection algorithm is used my almost every GPS receiver on the market in order to acquire the CDMA signals. One can extend the acquisition threshold to lower levels by lengthening the pre-detection integration (PDI) interval at the expense of acquisition time. Even so, there is a maximum PDI of about 10 milliseconds (100 Hz bandwidth) beyond which the sequential detection process breaks down. This is because the GPS signal structure includes BPSK modulated navigation data (50 BPS) transmitted on top of the 1.023 MHz spreading code that ultimately limits how long one can coherently integrate in order to increase the SNR. Beyond 10-20 ms (one data bit time), the data bit transitions cause the integration sum to be reduced or go to zero, depending on the phase relationship of the integration period relative to the data bit transition.
Also, contemporary Global Positioning System (GPS) receivers are often embedded within portable devices where energy is derived from a battery. These portable devices include devices such as cellular radiotelephones, PDAs (Personal Digital Assistants), portable computers, surveying devices and other devices that make use of information provided by a GPS receiver. When these GPS receivers operate, they consume a substantial amount of energy, which depletes energy from the battery that could be made use of by the co-embedded functions. If GPS correlation can be done faster, battery energy can be conserved because the GPS receiver can be turned off when correlation is achieved. Prior art schemes have inadequately addressed energy conservation.
What is needed is an improved GPS signal acquisition method and system that can operate with weaker signals and lock onto satellite signals faster than prior art schemes particularly for E911 calls.